common characteristic of G-protein coupled receptors (GPCRs) is the loss of physiological responsiveness following prolonged agonist exposure or desensitization. In many cases GPCR desensitization is mediated by a well- conserved mechanism involving receptor phosphorylation, internalization, and sorting for degradation or recycling. Clinically, GPCR desensitization is thought to underlie cases of drug-induced tolerance. Although much is known about GPCR desensitization, less is known about GPCR desensitization in polarized cells such as neurons. In the central nervous system, GPCRs are distributed on presynaptic axon terminals, on the cell body, and on postsynaptic dendrites where they regulate excitability and signal transduction. The effects of subcellular distribution on desensitization are not known. The proposed studies will investigate agonist-induced desensitization of GPCRs at presynaptic and postsynaptic terminals in neurons, using whole cell electrophysiology, receptor binding assays, and molecular biology. Preliminary data demonstrates agonist- induced desensitization of the A1 adenosine receptor (A1 R) occurs along a different time course in presynaptic domains than the same receptor located postsynaptically. The working hypothesis is that the subcellular location of GPCRs affects agonist-induced desensitization of these receptors, such that receptors in one region will desensitize slower than receptors in a different region. Data will be collected using primary hippocampal neurons and brain slices. Whole cell recording will be done to measure physiological acctivity of receptors located at distinct domains, and receptor binding studies will be done to measure receptor internalization of neurons chronically exposed to agonist. These studies will provide the first detailed description of how subcellular localization affects GPCR desensitization in neurons.